The present invention relates to glass ceramics and, in particular, to marble-like glass ceramics which are used for interior and exterior materials in buildings.
U.S. Pat. No. 3,964,917 (Reference I) corresponding to JP-B-51-23966 (Tokuko-Sho 51-23966) discloses a marble-like glass ceramic which is obtained from a thermally crystallizable composition consisting essentially of 50-65 wt % SiO.sub.2, 3-13 wt % Al.sub.2 O.sub.3, 15-25 wt % CaO, and 2-10 wt % ZnO when heat treated to convert at least a portion of the surface layer to crystals of .beta.-wollastonite.
JP-B-53-39884 (Tokuko-Sho 53-39884 - Reference II) discloses a marble-like glass ceramic consisting essentially of 45-75 wt % SiO.sub.2, 1-13 wt % Al.sub.2 O.sub.3, 6-14.5 wt % CaO, 1-13 wt % Na.sub.2 O+K.sub.2 O, 0-20 wt % BaO and 0-18 wt % ZnO, 4-24 wt % BaO+ZnO and comprising .beta.-wollastonite as the major crystal.
Those known glass ceramics are low in color tone stability due to variation of the heat treatment condition. That is, if it is heat treated at a condition slightly apart from the optimum condition, the precipitated amount of crystals changes considerably so that the color tone is changed from a desired one. This makes the large-scaled production of glass ceramics difficult.
Korean Patent (KP-B-) 91-9979 (Reference III) discloses a marble-like glass ceramic which is obtained from a thermally crystallizable composition consisting essentially of 55-63 wt % SiO.sub.2, 5-10 wt % Al.sub.2 O.sub.3, 17-22 wt % CaO, and 0.5-3 wt % Li.sub.2 O when heat treated to convert at least a portion of the surface layer to crystals of .beta.-wollastonite.
The glass ceramic disclosed in Reference III is devitrifiable and is not good in meltability, so that it is difficult to form an article of the glass ceramic. Further, crystal amount is so much that it is difficult to produce the glass ceramic with a dark color.
JP-A-3-164446 (Tokukai-Hei 3-164446 - Reference IV) discloses a glass ceramic consisting essentially of 48-68 wt % SiO.sub.2, 0.5-17 wt % Al.sub.2 O.sub.3, 6-22 wt % CaO, 5-22 wt % R.sub.2 O (R: Na, K), 0.2-8 wt % MgO, 0-6 wt % B203, 0-8 wt % BaO, 0-9 wt % ZnO, less than 15 wt % BaO+ZnO, and 0-10 wt % coloring agents and comprising .beta.-wollastonite as the major crystal.
The glass ceramic disclosed in Reference IV is devitrifiable and is not good in the color tone stability.